Make sure OffScreenCanvas API does not render moot our canvas fingerprinting protection

There is a new OffScreenCanvas API that allows running WebGL in web workers (e.g.). This is currently preffed of (ESR45) (gfx.offscreencanvas.enabled is set to false). But this sounds scary enough that we want to make sure that

a) preffing off the feature renders it really unusable (we should do this for esr45)

and

b) if it is preffed on our canvas fingerprinting protections are not harmed (can be checked when we transtions to an esr with this feature enabled).